db2诊断系列之---定位锁等待问题 
作者：tacy lee

 
在应用中，我们经常会碰到sql执行很慢，但是数据库cpu和内存使用率又不高的情况，类似的问题基本上由于锁，排序等原因造成，本文主要描述如何去定位锁等待问题，谁在锁等待？等待谁持有的锁？锁在那个表？

 
一、测试准备

 
1、先在session1执行如下操作，创建测试表

 
#db2 connect to eos
#export DB2OPTIONS=+C
#db2 "create table tacy_test (a int not null primary key,b varchar(10))"
#db2 "insert into tacy_test values(1,'a')"
#db2 "insert into tacy_test values(2,'a')"
#db2 "insert into tacy_test values(3,'a')"
#db2 "insert into tacy_test values(4,'a')"
#db2 commit
2、在session2执行如下操作

 
#db2 connect to eos
#export DB2OPTIONS=+C
二、产生一个lock wait

 
在session1做一个表更新：

 
#db2 "update tacy_test set b='b' where a=4"
sql执行成功
在session2做同样更新操作：
#db2 "update tacy_test set b='c' where a=4"
进程被挂起等待

 
三、定位锁等待

 
1、先来看看应用的情况：

 
#db2pd -db eos -applications

 
Database Partition 0 -- Database EOS -- Active -- Up 0 days 07:37:37

 
Applications:
Address    AppHandl [nod-index] NumAgents  CoorPid    Status                  C-AnchID C-StmtUID  L-AnchID L-StmtUID  Appid                           
0x10140040 8        [000-00008] 1          8425       Lock-wait               80       2          66       1          *LOCAL.db2inst1.071124043739    
0x100CE540 7        [000-00007] 1          8358       UOW-Waiting             0        0          80       2          *LOCAL.db2inst1.071124043708    

 
可以看到有一个应用的状态处于Lock-wait

 
2、现在我们来看看应用在等什么

 
#db2pd -db eos -locks showlock wait

 
Database Partition 0 -- Database EOS -- Active -- Up 0 days 07:42:56

 
Locks:
Address    TranHdl    Lockname                   Type       Mode Sts Owner      Dur HldCnt     Att Rlse
0x2C8E0760 3          02001806078066020000000052 Row        ..X  W   2          1   0          0   0x0  TbspaceID 2 TableID 1560 RecordID 0x2668007

 
锁的类型为Row（行锁），X锁（排他锁），下面是我们最关心的锁的位置

 
TbspaceID 2 TableID 1560 RecordID 0x2668007

 
其中TbspaceID为表空间ID，TableID为表的ID，RecordID代表具体位置，全部应该是0x0266807,其中前面三个字节为page number，为0x026680，后面一个字节代表solt identifier，为0x07 
3、找到相应的表

 
#db2 "select tbspace,tabschema,tabname,tableid,tbspaceid from syscat.tables where tbspaceid=2 and tableid=1560"

 
TBSPACE       TABSCHEMA   TABNAME    TABLEID TBSPACEID
------------  ----------- ---------- ------- ---------
USERSPACE1    DB2INST1    TACY_TEST     1560         2

 
1 record(s) selected.

 

 
4、根据RecordID找到锁在哪行

 
db2提供了一个强大的数据分析工具db2dart，可以dump出相应的page数据

 
#db2dart eos /dd /tsi 2 /oi 1560 /ps 157312p /np 1 /v y

 
Warning: The database state is not consistent.

 
Warning: Reorg rows MAY be due to the inconsistent state of the database.
DB2DART Processing completed with warning(s)!
Complete DB2DART report found in:
/home/db2inst1/sqllib/db2dump/DART0000/EOS.RPT

 
其中tsi为表空间id(2)，oi为表id(1560)，ps为page number(0x0266807)，需要转换为十进制，在结尾必须加p，np代表你要获取的页数，v为是否详细输出

 
现在我们来看看EOS.RPT

 
______________________________________________________________________________

 
_______                    DART                   _______ 

 
D a t a b a s e   A n a l y s i s   a n d   R e p o r t i n g   T o o l

 
IBM    DB2    6000
______________________________________________________________________________

 
DART (V8.1.0)  Report:
2007-11-24-20.59.51.355893

 
Database Name: EOS
Report name: EOS.RPT
Old report back-up: EOS.BAK
Database Subdirectory: /opt/db2/db2inst1/NODE0000/SQL00001
Operational Mode: Database Inspection Only (INSPECT)

 
______________________________________________________________________________
------------------------------------------------------------------------------

 

 
Action option: DD 
Table-object-ID: 1560; Tablespace-ID: 2; First-page: 157312p; Number-pages: 1; Verbose: y

 
Warning: The database state is not consistent.

 
Warning: Reorg rows MAY be due to the inconsistent state of the database.
Connecting to Buffer Pool Services...

 
Table object report phase start.
Dump format is verbose.

 
______________________________________

 
Page 0 of object 1560 from table space 2.

 
BPS Page Header:

 
Page Data Offset = 48
Page Data Length = 4048
Page LSN = 0000 AE97 AE41
Object Page Number = 0
Pool Page Number = 157312
Object ID = 1560
Object Type = Data Object

 
Data Page Header:

 
Slot Count = 8
Total Free Space = 2784
Total Reserve Space = 0
Youngest Reserve Space = n/a
Youngest TID = n/a
Free Space Offset = 2799
Maximum Record Size = 23

 
Data Records:

 

 
Slot 0:

 
Offset Location = 3996  (xF9C)
Record Length = 32  (x20)

 
Record Type = Data Object Header Control Record

 
Page count = 1
Object Creation LSN = 0000 AE97 800C
Object State = x0000
UDI Since Runstats = 0
DART Field = x00000000

 
Slot 1:

 
Offset Location = 2992  (xBB0)
Record Length = 1004  (x3EC)

 
Record Type = Free Space Control Record

 
Free space entries:
0:  2884 (x0B44),  4028 (x0FBC),  4028 (x0FBC),  4028 (x0FBC)
4:  4028 (x0FBC),  4028 (x0FBC),  4028 (x0FBC),  4028 (x0FBC)
8:  4028 (x0FBC),  4028 (x0FBC),  4028 (x0FBC),  4028 (x0FBC)
省略。。。
492:  4028 (x0FBC),  4028 (x0FBC),  4028 (x0FBC),  4028 (x0FBC)
496:  4028 (x0FBC),  4028 (x0FBC),  4028 (x0FBC),  4028 (x0FBC)

 
Slot 2:

 
Offset Location = 2916  (xB64)
Record Length = 76  (x4C)

 
Record Type = Table Directory Record

 
MetaIndex Root Page = 157377
Index Type = 2
Table Descriptor Pointer  --  Page 157312  Slot 3
Max Insert Search = 0
Flags = x02000200
bit representation = 00000010 00000000 00000010 00000000
Check pending info:
Constraint status    = x00
Constraint RID       = Page 0 Slot 0
last BID          = x00000000

 
Slot 3:

 
Offset Location = 2892  (xB4C)
Record Length = 24  (x18)

 
Record Type = Table Description Record

 
Number of Columns = 2

 

 
Column 1:
Type is Long Integer
Length = 4
Prohibits NULLs
Prohibits Default
Fixed offset: 0

 
Column 2:
Type is Fixed Length Character String
Length = 10
Allows NULLs
Prohibits Default
Fixed offset: 4

 
Slot 4:

 
Offset Location = 2869  (xB35)
Record Length = 23  (x17)

 
Record Type = Table Data Record (FIXEDVAR)

 
Fixed part length value = 15

 
Column 1:
Fixed offset: 0
Type is Long Integer
Value = 1

 
Column 2:
Fixed offset: 4
Type is Fixed Length Character String
61202020 20202020 2020                 a               

 

 
Slot 5:

 
Offset Location = 2846  (xB1E)
Record Length = 23  (x17)

 
Record Type = Table Data Record (FIXEDVAR)

 
Fixed part length value = 15

 
Column 1:
Fixed offset: 0
Type is Long Integer
Value = 2

 
Column 2:
Fixed offset: 4
Type is Fixed Length Character String
61202020 20202020 2020                 a               

 

 
Slot 6:

 
Offset Location = 2823  (xB07)
Record Length = 23  (x17)

 
Record Type = Table Data Record (FIXEDVAR)

 
Fixed part length value = 15

 
Column 1:
Fixed offset: 0
Type is Long Integer
Value = 3

 
Column 2:
Fixed offset: 4
Type is Fixed Length Character String
61202020 20202020 2020                 a               

 

 
Slot 7:

 
Offset Location = 2800  (xAF0)
Record Length = 23  (x17)

 
Record Type = Table Data Record (FIXEDVAR)

 
Fixed part length value = 15

 
Column 1:
Fixed offset: 0
Type is Long Integer
Value = 4

 
Column 2:
Fixed offset: 4
Type is Fixed Length Character String
61202020 20202020 2020                 a               

 

 
Slots Summary:  Total=8,  In-use=8,  Deleted=0.

 
Table object report phase end.
______________________________________

 
DB2DART Processing completed with warning(s)!
Warning(s) detected during processing.
______________________________________

 
Complete DB2DART report found in:
/home/db2inst1/sqllib/db2dump/DART0000/EOS.RPT
_______    D A R T    P R O C E S S I N G    C O M P L E T E    _______

 
找到Solt 7 (0x07)，ok，你现在可以清楚的知道应用等待的Row为(4,a)

 

 
总结

 
通过上面的方法，我们简单描述了一个db2锁问题的定位方法，希望能给大家在分析和定位应用性能问题的时候起到一定的帮助
根据rid能查到该row数据
==========================
在程序出现问题的时候，锁等待往往得不到释放，就是那个application执行完毕都还hold住表，在这种情况下，我们需要尽可能多的提供信息给开发人员，帮助他们定位问题所在，确实这个很useful。

 
1，db2pd -d mall -locks show detail 查看到锁等待row所在的page和slot
2，根据数据库ROWID的组成，pages为640和slot为4，如果是 large tablespace，则rowid转换为整数为:640*65536+4
（补充：一般说来，RID在v8里面是4字节，在v9里面的large tablespace（默认）是6字节。4字节的版本是3字节page id加上1字节的slot。6字节版本是4字节page id加上2字节的slot
）
3，select * from ORDERS where rid(ORDERS)= 54758604808 with ur   根据rid能查到该row数据
------
这块为什么要乘以65536
大家可能听说过RID，也很困惑RID是什么咚咚（只知道是指向某一个record的，但是不知道具体是怎么指的）。
一般说来，RID在v8里面是4字节，在v9里面的large tablespace（默认）是6字节。4字节的版本是3字节page id加上1字节的slot。
6字节版本是4字节page id加上2字节的slot

这块是v9 ,large tablespace ,所以2的16次方=65536 ,也就是一页有65536个slot, 故当前的row对应的rowid为640*65536+4